1. Field of the Invention
The invention relates to a display method, and related display controlling device capable of on-screen display, and more particularly, to a display method, and related display controlling device utilizing a DRAM to perform the on-screen display.
2. Description of the Prior Art
On-screen display (OSD) can be primarily divided into two kinds of OSD. The first OSD is a graphic based OSD system. This OSD system stores the whole picture in a memory by pixels. When the picture is displayed, the OSD system reads all the pixels of the picture from the memory in order to drive a display panel to display the picture. In other words, even though the picture contains some items in common (for example, the picture may be a stream, and there may be a lot of the same characters A in the stream), repetitively storing and processing every single pixel is required. The result is that the same items are read and stored from the memory many times.
The above-mentioned graphic based OSD system wastes enormous memory space by storing the same things. Therefore, a second OSD system, the font based OSD system, has been developed. The font based OSD system utilizes a block as a unit, and can store reusable font blocks in the memory. The font based OSD system can store a memory address of each block and block index (font index) in a look-up table. For example, the font based OSD system can store the picture block of characters A-Z in the memory and the corresponding memory addresses of the characters A-Z. Therefore, if the font based OSD system has to display a character A in a block of a display screen, the font based OSD system only has to input the font index of the character A, and the font based OSD system can access the corresponding memory address of the character A through the look-up table. Then, the font based OSD can read the picture of the character A. As mentioned above, the font based OSD system can re-use the block picture stored in the memory. In contrast to the graphic based OSD system, the font based OSD system saves memory space.
Generally speaking, the font based OSD system stores OSD data in a static random access memory (SRAM). Because the SRAM has characteristics of quick access, the OSD system can access the font data quickly. It is well known, however, that the SRAM requires lots of transistors and thus occupies a significant chip area for OSD.
Normally, a system chip is coupled to a dynamic random access memory (DRAM), which is addressed by a row address, a column address, and a bank address. In addition, if two data are read from or stored in the DRAM and the two data associate with different row addresses, many memory cycles are required to access the DRAM.
Please refer to FIG. 1, which is a timing diagram of accessing two data from the DRAM. As shown in FIG. 1, the data DATA0 and the data DATA1 respectively associate with the row address R0 and the row address R1. Therefore, accessing the two data DATA0 and DATA1 comprises the following steps:
1. Activate the memory space corresponding to the row address R0 of the DRAM;
2. Access the data DATA0 from the memory space corresponding to the row address R0;
3. Pre-charge the memory space corresponding to the row address R0;
4. Activate the memory space corresponding to the row address R1 of the DRAM; and
5. Access the data DATA1 from the memory space corresponding to the row address R1;
As mentioned above, as long as the two data are stored in different row addresses R0 and R1, the above-mentioned steps are performed in order to access the two data. In addition, with the data DATA1 displayed, assume that another data DATA2 stored in the row address R0 also needs to be displayed on the screen. Because the data DATA1 is stored in the row address R1, the OSD system still has to perform many steps to switch to the row address R0 in order to read the data DATA2. This process is therefore very complex, and wastes enormous memory bandwidth, which results in a poor accessing performance. In some cases where the DRAM is frequently accessed, the prior art OSD system cannot utilize DRAM to store the font data.